1. Field of the Invention
This invention relates to a method for forming a patterned resist layer on a semiconductor body. In other words, this invention relates to a process for the processing of negative-working resists suitable for lithography using energy radiation or a ray, especially, an electron beam, X-rays or an ion beam. Such a method comprises a step of heating a layer of negative-working resist on a semiconductor body in an atmosphere which is free from any oxidizing gases, after exposure to energy radiation and before development. It is particularly useful in the manufacture of semiconductor devices.
2. Description of the Prior Art
Electron-beam exposure systems have been found to be useful in providing controlled line exposure of submicron dimensions. Such dimensions, accompanied by strict tolerances, are essential to the production of integrated circuit chips. The procedure for such production is to expose a resist layer, applied to a suitable substrate or base material, usually a semiconductor, to an electron beam having a width of submicron dimensions. After the resist is developed, the substrate, not protected by the resist, is etched, either by a chemical solution or by a plasma beam.
As is well-known in the art, a variety of negative-working resists are useful for recording an electron beam in the electron-beam exposure devices. This is because they possess high sensitivity to electron beams of conventional voltage and also have submicron resolution capability, thermal stability, and high resistance to etching. After electron-beam exposure, the negative resists are cross-linked in the exposed area through a formation of free radicals and become insoluble in the developer used in the subsequent developing step. During development, the unexposed area of the resists is dissolved in the developer and, therefore, a negative image or resist pattern corresponding to the pattern of the electron-beam exposure is formed on the substrate.
It has been found that many or substantially all of the negative-working resists for electron-beam exposure, such as poly(glycidyl methacrylate) (PGMA), poly(glycidyl methacrylate-co-ethyl acrylate) (P(GMA-co-EA)), and poly(diallyl orthophthalate) (PDOP), cross-link after the electron-beam exposure while still in the electron-beam exposure device. The additional cross-linking reaction of the negative resists after the electron beam exposure is known in the art as "post-polymerization" and is considered to be caused by the radicals remaining in the resists after completion of the electron beam exposure. The post-polymerization in the exposed area of the resists results in undesirable variation of the width and thickness of the resulting resist patterns.
The post-polymerization of the electron-beam exposed negative-working resists is particularly prone to occur in device chips or parts of resist coated wafers, namely, semiconductor substrates. Resist coated wafers comprise a plurality of device chips. The first chip exposed is contained in the electron-beam exposure device, and therefore maintained under cross-linking conditions, until the electron-beam exposure of the last chip of the wafer is completed. The last chip exposed does not exhibit any, or in any case, exhibits less post-polymerization because the wafer is removed from the exposure device immediately after the completion of exposure of the last chip. The post-polymerization of the resists clearly depends on the residence time in the electron-beam exposure device after electron-beam exposure, and therefore decreases from the first to last electron-beam exposure steps. Actual wafers show relatively wide and thick resist patterns formed on the first chip exposed, and relatively narrow and thin resist patterns formed on the last chip exposed. Further, the percentage of residual coating increases along with the increased residence time of the exposed resists, in the exposure device. The term "percentage of residual coating" used herein means the percentage of the thickness of the coated resist left after electron-beam exposure and developing (residual resist coating), based on the initial thickness of the coated resists.
To solve the problem of the variation of width and thickness, it has heretofore been proposed to further maintain the electron-beam exposed negative resists in a curing chamber appended to the electron-beam exposure device, before developing, in order to intentionally cause a further cross-linking reaction of the exposed and thus cross-linked resists. The curing chamber must be in vacuum or filled with a non-oxidizing atmosphere such as nitrogen gas. This proposal is based on the fact that further or additional cross-linking of the resists in the curing chamber makes variations of the post-polymerization of the resists in the exposure device negligible and thus is effective for attaining a predetermined percentage residual coating.
One difficulty with the above procedure is that the curing time is relatively long, generally, about three to five times the electron-beam exposure time. This means that, to maintain the same speed of processing as without this additional step, three to five wafers must be processed in the curing chamber at once. This procedure is technically troublesome.